The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
The present disclosure generally relates to acoustic well logging, sometimes referred to as sonic well logging, and may include methods and systems for acoustic log data processing. The oil and gas industry uses various tools to probe formations penetrated by a borehole to locate hydrocarbon reservoirs and to determine the types and quantities of the hydrocarbons. Among these tools, acoustic tools have been found to provide valuable information regarding formation properties. In acoustic logging, a tool is lowered into a borehole and acoustic energy is transmitted from a source into the borehole and the formation. The acoustic waves that travel in the formation are then detected with an array of receivers.
Modern acoustic tools generally have multipole sonic sources. The multipole sources may include one or more monopoles, dipoles, and/or quadrupoles to excite different modes. Monopole mode excitation is traditionally used to generate compressional and shear head waves. From the monopole measurements, formation compressional and shear slowness can be obtained by processing the head wave components. However, in slow formations, which are defined as having a shear slowness higher than the borehole fluid slowness, the shear head waves are not measurable. Therefore, in slow formations, shear wave logging depends on borehole modes, such as dipole modes for wireline tools or quadrupole modes for logging-while-drilling (LWD) tools, to indirectly provide the formation shear slowness.
Unlike monopole head waves, the dipole or quadrupole borehole modes are dispersive. The characteristics of the dipole or quadrupole modes depend on formation shear slowness (DTs) as well as many other borehole-formation parameters, such as formation compressional slowness (DTc), formation density (ρb), mud slowness (DTm), mud density (ρm), and hole diameter (HD). Thus, to derive formation shear slowness (DTs) from the dipole or quadrupole modes, knowledge of these other borehole-formation parameters is provided.
A method for processing the dipole or quadrupole dispersive wave components to obtain the shear slowness is the dispersive slowness-time-coherence (DSTC) method disclosed in U.S. Pat. No. 5,278,805 issued to Kimball (assigned to the present assignee and incorporated herein by reference in its entirety). See also, Kimball, Geophysics, Vol. 63, No. 2, March-April 1998, pg. 337-344 and pg. 345-353. The DSTC method assumes most borehole-formation parameters, except DTs, are known. These borehole-formation parameters may be obtained from other logging operations or from the known dimensions of the borehole and the tool.
In general, the bandwidth for data telemetry for downhole tools may be limited, especially for Logging While Drilling (LWD) applications. As a result, most conventional sonic tools store received waveform data in memory downhole, and the data is retrieved and processed using surface computers. In such a case, the slowness data is only available after a job is complete. Even in wireline applications, the telemetry bandwidth is too limited to send all of the received waveform data to the surface to perform real time.